1. Field of the Invention
The present teaching relates to a profile measuring apparatus, a method for measuring a profile of an object, and a method for manufacturing a product. In particular, the present teaching relates to a profile measuring apparatus, a profile measuring method and a manufacturing method, which are configured to improve measurement precision.
2. Description of the Related Art
Conventionally, profile measuring apparatuses have been known for measuring the profile of an object by the shape from focus method (for example, see Japanese Patent No. 3321866). In a profile measuring apparatus utilizing the shape from focus method, for example, a projection optical system projects an image of a predetermined pattern on the object, and an optical imaging system takes an image of the object on which the pattern image is projected (to be referred to as an observational image hereinafter) while moving the object in an optical-axis direction of the optical imaging system. Then, the profile of the object is measured by calculating degree of focus (focus measure) of each pixel of a plurality of obtained observational images, and detecting the focus position of the pattern image based on the calculated degree of focus. Further, the degree of focus is a value denoting a state of focalization of the pattern image (at the focal point).
In the profile measuring apparatus utilizing the shape from focus method, the precision of the measurement becomes higher when the NA (numerical aperture) of the optical imaging system is larger, and when the focal depth of the optical imaging system is shallower.
FIG. 10 is a graph showing changes in the degree of focus of a certain pixel of observational images taken by optical imaging systems with different NA while moving the object in an optical-axis direction of the optical imaging systems. Further, the horizontal axis of FIG. 10 represents the position of the object in the optical-axis direction of the optical imaging systems, whereas the vertical axis represents the degree of focus. Further, the curve 501 shows a change in the degree of focus when utilizing the optical imaging system with a large NA, whereas curve 502 shows a change in the degree of focus when utilizing the optical imaging system with a small NA. As shown in this graph, because a large NA of the optical imaging system produces a rapider change in the degree of focus, the accuracy of detecting a focus position is improved. As a result, the precision of measuring the profile of an object is improved.